Guided waves (“GWs”) have long been of scientific and practical interest. GWs include surface-plasmon-polariton (“SPP”) waves and electromagnetic waves that propagate within waveguides, such as optical fibers and other dielectric or semiconductor waveguides. A SPP is an electromagnetic excitation with an electromagnetic field that propagates along an interface between a material with a negative dielectric constant, such as a metal, and a medium having a real, positive dielectric constant. A SPP is generated as a result of coupling a photon to a surface plasmon of the material with the negative dielectric constant. A plasmon is a quantum of the collective excitation of free electrons in a solid.
As shown in FIG. 1, a SPP wave 100 may be excited in a metal 102 having a dielectric constant ∈m by irradiating the metal 102 with electromagnetic radiation using a number of different illumination configurations. The SPP wave 100 propagates along an interface 102 between the metal 104 and surrounding dielectric medium 106 (e.g., air) having a real, positive dielectric constant ∈s. Because the SPP wave 100 is concentrated at the interface 102, the intensity of the SPP wave 100 may be two to three times the intensity of the electromagnetic radiation used to excite the SPP wave 100. Due to Ohmic losses in the metal 104, the intensity of the SPP wave 100 decays exponentially during propagation along the interface 102.
Free-space light cannot be directly coupled into surface plasmons of the metal 102 due a mismatch between the dispersion relations for the SPP wave 100 and incident photons. In other words, the free-space light and the surface plasmons exhibit different wave momenta at the same frequency. A number of different excitation configurations have been developed to couple free-space light into surface plasmons of the metal 104. One of the more common SPP excitation configurations is the so-called Kretschmann geometry in which a prism is used match the photon and SPP wavevectors. Some other common SPP excitation configurations include forming a diffraction grating in the metal 104 or roughening the interface 102 of the metal 104 to provide a similar diffraction effect.
The enhanced intensity of a SPP wave may be used in a number of different applications. For example, surface enhanced Raman spectroscopy (“SERS”) is a well-known spectroscopic technique for performing chemical analysis. In SERS, high-intensity electromagnetic radiation irradiates a specially prepared, nanostructured metal surface. A sample to be analyzed is placed on or near the roughened metal surface. Irradiation of the sample and the roughened metal surface generates an intense SPP that the sample experiences. The intense SPP is one factor that increases the number of Raman scattered photons from the sample that are characteristic of the sample's chemical composition. Raman scattered photons are a result of photons (i.e., Stokes and anti-Stokes radiation) that are scattered inelastically from the sample.
In addition to SPPs utility in sensor and spectroscopic applications, metallic interconnects that support SPPs are currently being investigated as replacements for conventional optical interconnects, such as optical fibers and other dielectric waveguides, used in electronic devices. As the size of electronic devices continues to relentlessly decrease every few years, further increases in processor speed may be prevented by thermal and signal delay issues associated with electronic interconnection between electronic components. Optical interconnects are believed to provide one solution to signal delay problems because optical interconnects posses a large data carrying capacity compared to conventional microscale or submicroscale metal signal lines. However, widespread utilization of optical interconnects has been hampered due to a large size mismatch between nanoscale and microscale electronic components and the optical interconnects. Optical interconnects are limited in size by the fundamental laws of diffraction to about half a wavelength of light and tend to be about one or two orders of magnitude larger than nanoscale and microscale electronic components.
Replacing conventional optical and metallic interconnects with plasmonic structures has been proposed because metals commonly used in electrical interconnects, such as copper and aluminum, allow excitation of SPPs. SPP waves propagating along a plasmonic interconnect could be used to transmit data signals to and from other chips or electronic devices. Thus, plasmonic interconnects would allow for the large data carrying capacity of conventional optical interconnects, while having the nanoscale or microscale dimensions of conventional metal interconnects. Additionally, plasmonic interconnects can also allow for electrical signals to be transmitted concurrently with SPP waves to further increase processing speed.
Therefore, a need exists for developing improved nanostructures that enable coupling electromagnetic radiation to GWs supported by the nanostructures. Additionally, a need exists to reduce the size mismatch between waveguides and other components in electronic and optoelectronic devices.